Chapter 25, Analysis 25-1
25 Analysis
25.1 Introduction
The second of five types of aviation weather information discussed in this handbook are analyses. Analyses
of weather information are an enhanced depiction and/or interpretation of observed weather data. Prior to
the 1990s, most analysis charts were hand -drawn by forecasters. Today ’s analyses are automated, and
depending on the weather information provider ( e.g., the NWS, commercial weather services, and flight
planning services), the appearance and content of these analyses will vary.
This chapter will only focus on those analyses produced by the NWS and made available on various
websites, including the AWC, the WPC, the Ocean Prediction Center (OPC), and the AAWU.
For this handbook, analyses include the following:
• Surface Chart Analysis.
• Upper Air Analysis.
• Freezing Level Analysis.
• Icing Analysis [Current Icing Product (CIP)].
• Turbulence [Graphical Turbulence Guidance (GTG)] Analysis.
• Real-Time Mesoscale Analysis (RTMA).
Chapter 25, Analysis 25-2
We recognize that in today’s world, an increasing amount of analyzed weather information is available
from multiple sources. These sources provide a wide range of meteorological data, such as surface
observations, radar, and satellite imagery, and use advanced techniques to combine these inputs into
accurate, high-resolution weather analyses products. These high-resolution analyses products can provide
METAR-equivalent data (ceiling, visibility, temperature, dew point pressure, wind direction, wind speed)
across grid that extend well beyond airport locations.
Analyzed Surface Weather information refers to weather data from sources other than the currently
approved ground-based ASOS and AWOS weather reporting systems. Alternate sources may include:
• Model-derived, gridded estimates of current conditions,
• In situ observations from novel data sources, such as UAS or weather cameras, and
• Surface sensors that are currently not approved for producing METARs.
FAA Flight Standards has investigated the use of Analyzed Weather Information since 2015 as an
alternative source of weather information when elements are missing from ASOS and AWOS reports. In
May of 2024, Flight Standards completed a Safety Risk Management Panel (SRMP) to assess the risk of
using Analyzed Weather Information ( for all required weather elements) for Part 121 and 135 operations
when sensed weather is not available (i.e., no ASOS/AWOS sensor) or missing (i.e., incomplete weather
information from an ASOS/AWOS METAR). The Safety Risk Management Document (SRMD) concludes
that implementation of identif ied mitigations to address performance variability of the analyzed weather
information (to include latency and geographic variability), all hazards would be reduced to a low risk level,
equivalent to current operational standards in the NAS regarding Weather Information. Results of the
SRMP supports a path forward to enable the use of Analyzed Weather as an approved source of weather
information when weather information is not available from an ASOS or AWOS.
At terminal locations that provide METARs, occasionally the information required for flight operations is
missing and this impacts the ability to sustain operations. Improved back-up information from an approved
weather source is required for efficiency an d to sustain safe operations. In an effort to mitigate these
disruptions, since 2015, Flight Standards approved the use of NWS Real -Time Mesoscale Analysis
(RTMA) for operators to use temperature information at airports where an ASOS/AWOS has failed. In
early 2024, Flight Standards also approved the use of Altimeter Setting (pressure) information at airports
where an ASOS/AWOS has failed. More details on RTMA are in Section 25.7.
With the assumption that a METAR is representative of weather within five miles of the terminal area
where the METAR is taken, this equates to two percent of Alaska and three percent of the lower 48
contiguous United States having “Certified Weather” information from “Approved Sources” at airports
where a METAR is produced. Cer tified weather systems from approved sources are too expensive to fill
these gaps across the NAS. However, utilizing existing FAA certified sensors from approved sources with
these other various types of meteorological data from multiple sources could easily help fill this gap.
25.2 Weather Charts
A weather chart is a map on which data and analyses are presented that describe the state of the atmosphere
over a large area at a given moment in time.
The possible variety of such charts is enormous, but in meteorological history there has been a more or less
standard set of charts, including surface charts and the constant -pressure charts of the upper atmosphere.
Because weather systems are three dimensional (3D), both surface and upper air charts are needed. Surface
weather charts depict weather on a constant-altitude (usually sea level) surface, while upper air charts depict
weather on constant-pressure surfaces.
Chapter 25, Analysis 25-3
The NWS produces many weather charts that support the aviation community.
25.2.1 Weather Observation Sources
Weather analysis charts can be based on observations from a variety of data sources (see Figure 25-1),
including:
• Land surface [e.g., ASOS, AWOS, and the National Mesonet].
• Marine surface [e.g., ship, buoy, Coastal-Marine Automated Network (C-MAN), and tide gauge].
• Sounding [e.g., radiosonde, dropsonde, pibal, profiler, and Doppler weather radar Velocity
Azimuth Display (VAD) wind profile].
• Aircraft [e.g., AIREP s and PIREPs), AMDAR, and Aircraft Communications Addressing and
Reporting System (ACARS)].
• Satellite [e.g., GOES sensors that provide temperature, moisture, and wind (through cloud
movement)].
Note: Human observers can augment automated reports.
Figure 25-1. Weather Observation Sources
25.2.2 Analysis
Analysis is the drawing and interpretation of the patterns of various elements on a weather chart. It is an
essential part of the forecast process. If meteorologists do not know what is currently occurring, it is nearly
impossible to predict what will happ en in the future. Computers have been able to analyze weather charts
for many years and are commonly used in the process. However, computers cannot interpret what they
analyze. Thus, many meteorologists still perform a subjective analysis of weather charts when needed.
25.2.2.1 Analysis Procedure
The analysis procedure is similar to drawing in a dot -to-dot coloring book. Just as one would draw a line
from one dot to the next, analyzing weather charts is similar in that lines of equal values, or isopleths, are
drawn between dots representing various elements of the atmosphere. An isopleth is a broad term for any
line on a weather map connecting points with equal values of a particular atmospheric variable. See Table
25-1 for common isopleths.
Chapter 25, Analysis 25-4
Table 25-1. Common Isopleths
Isopleth Variable Definition
Isobar Pressure A line connecting points of equal or constant pressure.
Contour Line (also
called Isoheight) Height A line of constant elevation above MSL of a defined surface,
typically a constant-pressure surface.
Isotherm Temperature A line connecting points of equal or constant temperature.
Isotach Wind Speed A line connecting points of equal wind speed.
Isohume Humidity A line drawn through points of equal humidity.
Isodrosotherm Dewpoint A line connecting points of equal dewpoint.
The weather chart analysis procedure begins with a map of the plotted data , which is to be analyzed
(see Figure 25-2). It is assumed that bad or obviously incorrect data has been removed before beginning the
analysis process. At first, the chart will appear to be a big jumble of numbers. However, when the analysis
procedure is complete, patterns will appear, and significant weather features will be revealed.
25.2.2.1.1 Step 1: Determine the Optimal Contour Interval and Values to be Analyzed
The first step in the weather chart analysis procedure is to identify the maxima and minima data values and
their ranges to determine the optimal contour interval and values to be analyzed. The best contour interval
will contain enough contours to identify significant weather features, but not so many that the chart becomes
cluttered. Each weather element has a standard contour interval on NWS weather charts, but these values
can be adjusted in other analyses as necessary.
Figure 25-2. Analysis Procedure Step 1: Determine the Optimal Contour Interval and Values to be Analyzed
Chapter 25, Analysis 25-5
Every contour value must be evenly divisible by the contour interval. So, for example, if the contour interval
is every four units, a 40-unit contour is all right, but a 41-unit contour is not. In the surface pressure analysis
example shown in Figure 25-2, an isobar analysis will be performed beginning at a value of 992 mb and
using a contour interval of four mb, which is standard on the NWS Surface Analysis Chart.
25.2.2.1.2 Step 2: Draw the Isopleths and Extrema
The second step is to draw the isopleths and extrema (maxima and minima) using the beginning contour
value and contour interval chosen in the first step (s ee Figure 25-3). It is usually easiest to begin drawing
an isopleth either at the edge of the data domain (edge of the chart) or at a data point that matches the
isopleth value being drawn. Interpolation must often be used to draw isopleths between data points and
determine the extrema. “Interpolate” means to estimate a value within an interval between known values.
When drawing isopleths and extrema on a weather chart, certain rules must be followed:
1. The analysis must remain within the data domain. Analysis must never be drawn beyond
the edge of the chart where there are no data points. That would be guessing.
2. Isopleths must not contain waves and kinks between two data points. This would indicate
a feature too small to be supported by the data. Isopleths should be smooth and drawn
generally parallel to each other.
3. When an isopleth is complete, all data values must be higher than the isopleth’s value on
one side of the line and lower on the other.
4. A closed-loop isopleth must contain an embedded extremum (maximum or minimum).
5. When a maximum (minimum) is identified, data values must decrease (increase) in all
directions away from it.
6. Isopleths can never overlap, intersect, or cross over extrema. It is impossible for one
location to have more than one data value simultaneously.
7. Each isopleth must be labeled. A label must be drawn wherever an isopleth exits the data
domain. For closed-loop isopleths, a break in the loop must be created where a label can
be drawn. For very long and/or complex isopleths, breaks should be created where
additional labels can be drawn, as necessary.
8. Extrema must be labeled. Extrema are often denoted by an “x” embedded within a circle.
Beneath the label, the analyzed value of the field must be written and underlined.
9. Isopleths and labels should not be drawn over the data point values. If necessary, breaks
in the isopleths should be created so that the data point values can still be read.
Chapter 25, Analysis 25-6
Figure 25-3. Analysis Procedure Step 2: Draw the Isopleths and Extrema
25.2.2.1.3 Step 3: Identify Significant Weather Features
The third (and final) step is to interpret significant weather features. The conventional labels for extrema
are H (high) and L (low) for pressure and height, W (warm) and K (cold) for temperature (they stand for
the German words for warm and cold), and X (maxima) and N (minima) for all other elements. Tropical
storms, hurricanes, and typhoons are low-pressure systems with their names and central pressures denoted.
Troughs, ridges, and other significant features are often identified as well.
For surface analysis charts, positions and types of fronts are shown by symbols in Figure 25-4. The symbols
on the front indicate the type of front and point in the direction toward which the front is moving. Two short
lines across a front indicate a change in front type.
Table 25-2 provides the most common weather chart symbols. In the surface pressure analysis in Figure
25-4, a high, low, trough, and ridge have been identified.
1 1
1 4
1 4
Chapter 25, Analysis 25-7
Table 25-2. Common Weather Chart Symbols
Feature Symbol Definition
Low
A minimum of atmospheric pressure in two dimensions (closed
isobars) on a surface chart, or a minimum of height (closed
contours) on a constant-pressure chart. Also known as a cyclone.
High
A maximum of atmospheric pressure in two dimensions (closed
isobars) on a surface chart, or a maximum of height (closed
contours) on a constant-pressure chart. Also known as an
anticyclone.
Trough
An elongated area of relatively low atmospheric pressure or height.
Ridge
An elongated area of relatively high atmospheric pressure or
height. May also be used as reference to other meteorological
quantities, such as temperature and dewpoint.
Figure 25-4. Analysis Procedure Step 3: Interpret Significant Weather Features
25.2.3 Surface Analysis Chart
The WPC in College Park, MD, produces a variety of surface analysis charts for North America that are
available on their website. The WPC’s surface analysis is also available on the AWC’s and other providers’
websites.